1. Field of the Invention
The present invention relates to a method for measuring aberration, and particularly to an aberration measuring method for measuring wavefront aberration of an optical system such as a projection optical system for transferring a pattern on a mask onto a photosensitive substrate. Such a projection optical system is used for example, in a lithography process for exposing an article to be processed such as a single crystal substrate like a semiconductor wafer or a glass substrate used for a liquid crystal display (LCD).
2. Related Background Art
In the process for manufacturing microscopic semiconductor devices such as semiconductor memories or logic devices utilizing a photolithography techonology, a projection exposure apparatus for projecting a circuit pattern formed on a reticle or a mask (these terms will be interchangeably used in this specification) onto a wafer or the like by a projection optical system so as to transfer the circuit pattern has been conventionally used.
The projection exposure apparatus is required to transfer a pattern on a reticle at a predetermined magnification (i.e. a reduction ratio) precisely. In order for this requirement to be met, it is important to use a projection optical system that has a superior imaging performance with extremely reduced aberrations. In recent years particularly, with rapid miniaturization of semiconductor devices, patterns beyond ordinary imaging performance are often required to be transferred, and the transferred patterns have become sensitive to aberrations of optical systems. On the other hand, increases in the exposure area and the numerical aperture (NA) of the projection-optical system have been required. This makes aberration correction all the more difficult. In order to attain effective aberration correction, it is necessary to measure wavefront aberration with high precision.
As an apparatus for measuring wavefront aberration of an optical system with high precision, an apparatus utilizing a Fizeau interferometer or a Twyman-Green interferometer has been conventionally used. In the following, the principle of measuring the wavefront aberration of a projection lens equipped in a projection exposure apparatus as a lens to be measured using a Fizeau interferometer will be described with reference to FIGS. 7 to 9. FIG. 7 is a diagram schematically showing a conventional aberration measuring apparatus 1000.
Light emitted from a light source 1100 is guided to an interferometer unit 1200, transmitted through a half mirror 1210, converted into parallel light by a collimator lens 1220, transmitted through a TS lens 1300 and a lens to be measured 1400, and reflected by an RS mirror 1500. The light reflected by the RS mirror 1500 is made to pass through the lens to be measured 1400 and the TS lens 1300 in the opposite direction, then reflected by the half mirror 1210 and made incident on a CCD camera 1240 as light to be measured (or measurement light) by means of an imaging lens 1230.
On the other hand, the light reflected by the last surface (i.e. the Fizeau surface) of the TS lens 1300 is also reflected by the half mirror 1210 and made incident on the CCD camera 1240 as reference light by means of the imaging lens 1230. These two light fluxes (i.e. the measurement light and the reference light) interfere with each other, so that interference fringes are detected on the CCD camera 1240. The wavefront aberration can be determined by calculation based on the interference fringes. The TS lens 1300 and the RS mirror 1500 are scanned along the optical axis direction, so that the wavefront aberration can be measured continuously by the so-called fringe scanning method. The parts in FIG. 7 that are not designated by reference signs will be described later in the description of the embodiments, and so the description is omitted here.
An aperture stop 1410, which determines the numerical aperture of the lens to be measured 1400, is disposed at a position optically conjugate with the CCD camera 1240. This arrangement will be specifically described in the following with reference to FIGS. 8A and 8B. FIGS. 8A and 8B are block diagrams schematically showing the positional relationship between the aperture stop 1410 and the CCD camera 1240 shown in FIG. 7.
The aperture stop 1410 of the lens to be measured 1400 is conjugate with the front focal plane FP (on the interferometer unit 1200 side) of the TS lens 1300 with respect to the downstream optical system (that is, the lens system on the image plane side of the aperture stop 1410) 1600 of the lens to be measured 1400 and the TS lens 1300. In addition, the front focal plane FP of the TS lens 1300 is conjugate with the detection surface 1240a of the CCD camera 1240 with respect to an interference optical system (that is, the collimator lens 1220 and the imaging lens 1230 that constitute the interferometer unit 1200) 1700. To be precise, the TS lens 1300 is disposed at a measurement position on the axis and the position of the detection surface 1240a is adjusted in such a way that the aperture stop 1410 and the detection surface 1240a become optically conjugate with each other in the assembling and adjusting process.
Consequently, although the diameter of the aperture stop 1410 of the lens to be measured 1400 is equal to the effective numerical aperture of the lens to be measured 1400, diffracted light from the edge of the aperture stop 1410 does not affect the wavefront aberration detected based on the interference fringes, since the diffracted light is imaged on the detection surface 1240a. 
However, when the TS lens 1300 is displaced to an off-axis measurement position, as shown in FIG. 8B, the optically conjugate relationship between the aperture stop 1410 of the lens to be measured 1400 and the detection surface 1240a of the CCD camera 1240 is no longer kept. This is because the interference optical system 1700 is shifted relatively to the lens to be measured 1400 with the displacement of the TS lens 1300 and the distance between the TS lens 1300 and the interference optical system 1700 changes by ΔL (the shift amount of the TS lens 1300).
In the case that the aperture stop 1410 of the lens to be measured 1400 and the detection surface 1240a of the CCD camera 1240 are out of the optically conjugate relationship, the diffracted light will spread on the detection surface 1240a. In that case, a rapid change in the phase of the measured wavefront aberration will be caused in the periphery of the effective numerical aperture NA0 (i.e. the pupil) of the lens to be measured 1400 on account of an influence of the diffracted light from the aperture stop 1410. This will cause a considerable measurement error. Here, FIG. 9 is a diagram schematically showing the wavefront aberration in the periphery of the pupil of the lens to be measured 1400 in the conventional aberration measuring apparatus 1000.
Particularly, in the case that the measurement light is made incident from the object plane side as is the case with the aberration measuring apparatus 1000 shown in FIG. 7, the shift amount of the TS lens 1300 between the on-axis position and the off-axis position becomes larger as compared to the case in which the measurement light is made incident from the image side (for example, in the case of a projection lens of 5× magnification, the shift amount ΔL becomes 25 times larger). Consequently, an error of the wavefront aberration in the periphery of the pupil due to the spread of the diffracted light becomes larger.
On the other hand, it is possible to always keep the optically conjugate relationship between the aperture stop 1410 and the detection surface 1240a by shifting the CCD camera 1240 or the imaging lens 1230 along the optical axis in accordance with the measured image height. However, that method is not desirable, since the interference fringes will be shifted on the CCD camera 1240 on account of eccentricity generated upon shifting the CCD camera 1240 and correction needs to be carried out in the wavefront aberration calculation area for every image height of the central coordinate.